Mojanda is one of the largest volcanoes in the Ecuador Inter-Andean Depression, between the two cordilleras.  The 4,263 m complex is constructed of a pair of adjacent volcanoes, Mojanda and Fuya Fuya, 3 km apart.  The system is described currently as inactive.  It erupted andesites, basaltic andesites, dacites and rhyolites.

The ancestral volcano created a large caldera which was filled after formation by the current two stratovolcanoes.  The summit has a smaller caldera currently occupied by three crater lakes, Caricocha (Karikucha, Man Lake, Laguna Grande, Laguna Hombre), Yanakucha (Black Lake, Laguna Negra) and Huarmicocha (Warmikucha, Woman Lake).  The lakes are popular tourist destinations, a 45-minute taxi ride away from neighboring Otavalo. 

Due to the relatively high altitude at the lakes, the climate is cold, with an average temperature of 8° C.  Hikers should bring jackets, walking shoes, sunglasses and water.  The best time to visit is in the morning.  Water in the lakes is cold and as such, I have found no description of fishing, boating or swimming.  There do not appear to be any hot springs associated with the lakes.  Vegetation is described as Andean Paramo, with grasses and bushes.  The views from the surrounding volcanic rim across the lakes and across the surrounding plains to neighboring volcanoes are described as spectacular. 

16,000 live within 10 km of the volcano, 480,000 within 30 km, and nearly 3.1 million within 100 km, most of them in the capital city of Quito 38 km to the WSW.

The closest town is Otavalo, capital of Otavalo Canton, with around 40,000 inhabitants. The population is largely indigenous people. It is located in the valley floor between three surrounding volcanoes, Cuicocha – Cotacachi to the W, Cerro Imbabura immediately to the SE, and Mojanda immediately to the S. Both Cuicocha and Mojanda have associated crater lakes.

Otavalo is famous for colorful woven wool textiles sold in local shops. The most famous of these is the combined Saturday market. At its peak, almost a third of the town is full of stalls selling various handmade goods, spices and foods.

The town was originally a farming community taking advantage of the volcanic soils. Over the years, the popularity of the markets and handmade goods has grown to the point where tourism has become an important part of the local economy.  Local businesses cater to tourists and tours with local hotels, hostels, and tour operators. Neighboring towns specialize in other handmade goods such as leather and wood carving.

Volcano monitoring in Ecuador is done by Instituto Gerfisico under Escuela Politecnica Nacional.  They list 77 volcanoes in the country, six of which are actively monitored.  City-webcams.com lists at least three city webcams in Otavalo.  However, the site also throws a security warning when you try to browse to them, so be warned.  As far as I can tell, there are no dedicated webcams for Mojanda. 

Region

In recent years, we covered three volcanoes in Ecuador, El Reventador in 2015, Cotachi – Cuicocha in 2021, and Quilotoa in 2023.  The tectonics section in El Reventador was extensive and would be a good place to start for a more in-depth review of the region. 

The Ecuadoran Andes is divided into two chains, the Western and Eastern Cordillera, separated by the Interandean Depression.  There are at least 77 recognized volcanic centers distributed along the two cordilleras and in the depression.  They form a continuous volcanic belt 300 km long and 120 km wide.  27 of these volcanoes are considered active.  Seven of them have erupted since 1532.

The volcanic front along the Western Cordillera generally has a 35 km spacing between young volcanic centers stretching from Cuicocha in the north to Iliniza to the south.  South of Iliniza is an 85 km gap in volcanism before the next group of volcanoes, Carihuairazo and Chimborazo.  There is a similar gap in the Eastern Cordillera between Cotopaxi in the north and Tunguahua to the south.  The only active volcanic center in either of these gaps is Quilotoa, which is 20 km west of the main axis of the Western Cordillera.  The volcano is located at the intersection of an older inactive N-S trending fault system that controls the path of the Toachi river and a series of active NE-SW trending faults.

Given the large number of volcanoes in close proximity to one another in this part of Ecuador, we will limit our discussion of volcanoes to those within 30 km or so from Mojanda.  This includes most volcanoes ringing Quito, some 30 km SW.  Distances will be measured from the middle of Mojando’s Laguna Grande. 

Cotacachi – Cuicocha

We covered Cotacachi – Cuicocha in our 2021 post.  Cuicocha is located some 21 km NNW from Mojanda.  Cotacachi is located another 7 km farther N. 

Cusin

Cerro Cusin is a 3,989 m stratovolcano some 15 km WNW from Mojando.  It is one of multiple stratovolcanoes ringing Otavalo.  The 4 km diameter summit caldera is breached to the NW with most of the cone removed from the amphitheater.  The satellite view of the scar appears to be quite deep.  There is a second, smaller amphitheater parallel to the main one on the N whose source appears to be lower down the flank. 

Cayambe

Cayambe is a massive compound andesitic – dacitic stratovolcano located on the W edge of Ecuador’s Cordillera Real, E of the InterAndean Valley, 34 km SE from Mojando.  The 5,790 m volcano is capped by extensive glaciers.  The volcano is constructed from an older and a newer stratovolcano, with the newer summit 1.5 km E of the older one.  The newer summit has two lava domes 1.5 km apart.  Several other domes are on the upper flanks erupted pyroclastic flows down the lower flanks.  The La Virgen recent pyroclastic cone on the lower E flank erupted thick andesitic lava flows that traveled 10 km E.  Nevado Cayambe produced frequent explosive eruptions over the last 4 ka.  The most recent eruption was 1785 – 1786.  Smithsonian GVP carries bulletin reports of increased / anomalous seismic activity beneath the volcano in 2006 and 2017. 

Pululahua

Pululahua is a 3,360 m, low, forested volcano some 27 km WSW from Mojanda.  It is topped with a 5 km caldera narrowly breached to the W and partly filled by a group of dacite lava domes.  Pre-caldera domes are found outside the caldera on the E, SE and S flanks.  The four post-caldera domes rise 450 m above the caldera floor.  Large explosive eruptions produced pyroclastic flows in the last 120 ka.  Caldera formation took place 2.7 ka.  The most recent eruption was some 300 AD.  It produced dacite lava and pyroclastic flows.

The caldera is notable for holding the Pululahua Geobotanical Reserve, one of two volcanic craters with both a town in it and farming.  It was first populated by the Incas likely not long after the 300 AD eruption. 

Pambamarca

Pambamarca is a 4,076 m Pleistocene stratovolcano located some 25 km S of Mojanda.  It is part of the volcanic ring around Quito, E of the N end of the city.  Pambamarca is notable for the largest concentration of Incan forts built before the arrival of the Spanish.  The war between indigenous tribes and the invading Inca was at least 17 years long, not settled until the 1500s.  Inca were subsequently conquered by the Spanish shortly after this war ended, making it a short-lived victory. 

Imbabura

Imbabura is the final member of the group of volcanoes ringing Otavalo, 17 km NE from Mojanda.  It is a 4,609 m stratovolcano with a dome complex on its SW flank.  Construction of the andesitic stratovolcano ended before 43 ka.  The cone suffered a major flank collapse putting a debris avalanche 16 km N.  Subsequent activity in the amphitheater scar grew and collapsed large dacite domes.  The flank collapse appears to be a hot collapse with a lateral blast.  Historic reports were mudflows and rockslides rather than active magmatism or phreatic explosions.  The most recent eruption was around 7.5 ka. 

Cubilche

The Cubilche volcanic complex is located 6 km SE from Imbabura, 18 km NE from Mojanda.  It is described as part of the greater Imbabura volcanic complex rather than an independent system as it has a common magma reservoir.  The complex tops out at 3,836 m and consists of three young, well preserved eruptive centers, one of them a dome complex.  There is also a scar that opens to the E formed during a dome collapse event and directed explosion.  The event created a 1.6 km3 debris avalanche that covered at least 80 km2, reaching at least 13 km from the source.  Post collapse activity erupted andesitic lavas and pumices.  There is a second smaller debris avalanche from a breach to the N.  The pyroclastic flow associated with the debris avalanche dates around 44 ka.  It was followed by explosive eruptions of scoria falls and block and ash flows 32 ka and 29 – 28 ka.  Most recent activity was strombolian and phreatomagmatic eruptions from the youngest crater now occupied by the small Cubilche Lake.  Scoria mantling the complex looks quite young in satellite view but is undated the best I can find out. 

Mojando – Fuya Fuya

The Mojanda volcanic complex is slightly elliptical, 25 km N-S and 30 km E-W.  It rises from a base at 2,200 – 3,000 m to 4,263 m at Cerro Fuya Fuya.  There is a small 2.5 x 3 km summit caldera occupied by Laguna Grande de Mojanda.  The basement rocks are ancient basalts and thick erosional deposits.  Lower slopes on the S, E and N are relatively gentle and covered with thick pyroclastic deposits.  W slopes are steeper, more uneven, and more heavily eroded with lava flows on the intermediate slopes.  There are a series of domes on the W side.

The complex was initially thought to be a single volcanic system.  Upon further review, two volcanoes were identified, separated by 3-4 km.  Mojanda is the older, eastern volcano, active 600 – 200 ka.  Fuya Fuya is the younger, western one.  Base lava flows at Mojanda are radially distributed.  There is the remains of a somma between 3,750 – 4,000 m marking an older caldera 5 km wide, now occupied by the younger summit.  The eruption that created that caldera has not been characterized.  The S part of the summit caldera of the younger Mojanda cone has a 1.5 km crater-like depression occupied by Laguna Negra.  This is thought to be the youngest volcanic structure of Mojanda activity.

The caldera wall is breached to the W and occupied by a complex of dacitic lavas and domes which make up the upper part of the Fuya Fuya edifice.  It is formed by a 6 km depression open and inclined to the W.  The depression demonstrates that the new volcano suffered a large flank collapse before dome emplacement.  The Cushnirumu Peak and associated mountains are remnants of an older, heavily eroded volcano.  This volcano has a collapse structure open to the SW suggesting it was largely destroyed by a flank collapse / debris avalanche event. 

There is a chemical difference between magmas erupted at Mojanda and Fuya Fuya.  Mojanda rocks appear to come from partial melting of the mantle wedge.  Its primary magmas are slightly evolved as shallow levels with some crystallization.  Fuya Fuya magmas appear to be some combination of mantle wedge melts mixing with slab melts and partial crystallization of multiple batches of melt in a shallow chamber.  

Mojanda construction

Initial activity built Lower Mojanda, an 18 km wide stratocone that likely topped out at 4,600 m, suggesting a volume of 140 km3.  This activity was largely effusive, erupting a thick sequence of andesitic lavas.  The initial cone was terminated by formation of the older 5 km diameter caldera.  There are extensive lahar deposits of dacite pumice in neighboring valleys from this structure.  Subsequent activity began with lava flows and became increasingly explosive with multiple lava flows, scoria flows, block and ash flows, breccia formation, and scoria falls.  Deposits from these events can be found at all altitudes on the slopes of the cone. 

Post collapse activity filled the caldera with thin lava flows and built a new cone 6-7 km wide that spread outside the caldera rim to the S and NE.  The new cone, Upper Mojanda (Mojanda II) cone has scorias, breadcrust bombs, quenched clasts and surge structures, suggesting strong explosive eruptions, an open conduit, and extensive magma-water / ice interactions.  Blocks in the deposits suggest dome construction, destruction, and periodic lava plugs in the conduit.

There is a thick sequence of pyroclastic flow and fall deposits that include two large rhyolite layers.  VOGRIPA carries a pair of VEI 4.0 eruptions that ejected 0.1 km3 of material from Mojanda.  The first was 350 ka.  The second, 200 ka.  The calderas filled with new domes and block and ash flows. Each of these cycles included multiple smaller cycles of dome building, dome destruction and block and ash flows. 

The end of this phase had less magma-water interaction, dome extrusion and block and ash flows.  Most of the eruptions toward the end of activity at Mojanda took place through snow and ice.  Final phreatomagmatic activity left a thick andesitic fall.  The snow / ice cap was likely destroyed by eruptions.  There are dry airfall deposits interbedded with surge and lahar deposits suggesting magma interaction with water / ice was highly variable and lasted for a long time.  This activity formed the current summit caldera.  Some eruptions were quite large, with ash found up to 30 km away.  This cone was partly destroyed by a highly explosive phreatomagmatic event which formed the youngest 3 km diameter summit caldera.

Fuya Fuya construction

During Upper Mojanda activity, new activity began on the W flank of Mojanda, emplacing Lower Fuya Fuya (Fuya Fuya I) lavas and domes.  Fuya Fuya I is a sequence of large andesitic to dacitic extrusions emplaced on the W flank of the base Mojanda stratovolcano.  Lava domes and flows extend up to 12 km SW from Fuya Fuya summit.  They do not appear N of the avalanche caldera.  Initial activity was dome building and destruction via repeated Plinian eruptions.  Lahars were active during this construction layering lahar deposits with Plinian pumice beds and block and ash flows. 

The second main construction phase of Fuya Fuya built the well-formed San Bartolo stratocone following the 200 ka caldera formation. It was centered 1.0 – 1.5 km S of the present summit.  The cone was 10 km wide, may have topped out at 4,600 m, with a volume of 40 km3, similar in size to neighboring Upper Mojanda cone.  This cone was mainly built of andesitic lavas but also has block and ash flows from explosive events. 

San Bartolo was partly destroyed by the flank collapse / debris avalanche event 126 ka which removed a large part of the cone and the W flank of Lower Mojanda.  The sector collapse to the W created a major avalanche deposit below the volcano and created a large amphitheater in the summit.  The collapse was a hot event, with voluminous dacitic ash flows, airfalls and surges immediately followed the avalanche, similar to that of Mount St Helens.  Most of these were directed W.  Prevailing winds carried large quantities of ashfall N. 

The avalanche followed valleys W until stopped by the deep Guayllabamba canyon.  It flowed around the Puellaro dome, filled and dammed the Guayllabamba River canyon, creating a lake over 120 m deep covering 125 km2.  Subsequent activity erupted ash flows N and W.  Surge deposits cover at least 50 km2 and cover avalanche deposits 2-3 m to 20 m thick in depressions. 

Activity then shifted to the strongly explosive Upper Fuya Fuya (Fuya Fuya II).  A new composite cone 5 km wide, some 1,000 m high grew within the avalanche amphitheater.  It is constructed of thick lava flows and domes.  Lava flows traveled up to 14 km from the vent in the amphitheater.  Explosive activity resumed with pyroclastics and block and ash flows, layered surges, airfalls and lahars.  Block and ash flows were directed down the amphitheater to the W.  Other smaller ash flows traveled S.  Pyroclastic falls covered the combined summit.  The most recently dated ashflow dates older than 35 ka. 

Three central domes and two satellite domes, Colangal and Panecillo are considered to be the last extrusions of this activity.  They do not appear to be glaciated and were likely extruded sometime in the last 10 ka. 

Tectonics

The tectonics section in our 2015 El Reventador post was extensive and would be a good place review regional tectonics.  I will be cross posting a short, edited version of that treatment. 

Volcanic activity in Ecuador is driven by the subduction of the Nazca Plate under the South American Plate, creating the Andes. This portion of the Northern Andes is segmented into 125 km-long chunks. Most of the segment boundaries appear to be unrelated to the structure of the subducted plate.

Distance from the subduction trench to volcanic front is highly variable. The northern segment in Columbia is over 380 km E of the trench, and the volcanoes are 140 – 160 km above the subducted slab. Distance between the trench and volcanic front decreases to around 300 km in southern Ecuador, with the volcanoes only 80 – 100 km above the slab. The arc varies in thickness from a single chain of volcanoes in northern Columbia to a broad belt 80 – 120 km wide in Ecuador. Ecuador also has a line of alkaline back-arc centers another 50 km to the E. Crust thickness here varies from 40 – 55 km.

The Carnegie Ridge started subducting beneath South America at least 8 – 2 Ma. This subducted ridge may extend 110 – 500 km E from the trench, meaning that subduction of it may have begun far earlier that currently believed.  There appears to be two tears in the subducting slab bounding the subducting Carnegie Ridge. The ridge tends to shallow out and subduct as a flat plate underneath South America. The portions of the Nazca Plate north and south of the tears subduct more steeply, at the 25 – 30-degree angle.

The Carnegie Ridge thickness itself is thought to be larger than 17 km thick, which is in line with its source, the Galapagos hotspot to the west. It is also buoyant.  An alternate theory has the subducting portion of the Carnegie Ridge detaching and sinking into the mantle around 100 km from the trench, creating a slab window. This is supported by an observed seismic gap at intermediate depth. The 1998 Gutscher, et al paper this comes from concludes the two-tear description is the more likely description of what is actually going on.

Conclusions

While activity at Mojanda – Fuya Fuya has waned over the last few thousand years, there is certainly magma available in the region, most recently at Cubilche, 18 km NE.  The lack of an active hydrothermal system also indicates a dormant state.  The region is dotted with multiple closely spaced volcanoes.  While there is no immediate obvious danger from this system, the subduction continues making magma available.  As with most volcanoes, dormancy here may end up being a temporary state rather than a permanent one. 

Additional information

Mojanda volcanic complex (Ecuador):  development of two adjacent contemporaneous volcanoes with contrasting eruptive styles and magmatic suites, Robin, et al, Dec 1997

The Chota Basin and its significance for the inception and tectonic setting of the Inter-Andean depression in Ecuador, Winkler, et al, May 2005

Chapter 13, Agricultural erosion in the Ecuadorian Andes, a natural and historical phenomenon, Land husbandry, components and strategy, FAO Soils Bulletin 70

Systematic time-controlled geochemical changes in the Ecuadorian volcanic arc, Andrade, et al, 2005

Adakites from Ecuador:  preliminary data, Monzier, et al, Sept 1996

Assessment of geomorphosites for geotourism in the northern part of the “Ruta Escondida” (Quito, Ecuador), Carrion-Mero, et al, Oct 2020

Evolution of the late Pleistocene Mojanda – Fuya Fuya volcanic comples (Ecuador), by progressive adakite involvement in mantle magma sources, Robin, et al, Apr 2009

Contrasted eruptive styles and magmatic suites (andesitic vs. adakitic) at Mojanda volcano, Ecuador, Robin, et al, Jan 1996

Geochemistry of Cayambe and Mojanda-Fuya Fuya volcanic complexes:  evidences for slab melts-mantle wedge interactions, Samaniego, et al, Jan 1999

The Cubilche volcanic complex, Imbabura province, Ecuador:  a first investigation of its evolution and petrology, Navarrete, et al, Sept 2019